It has been known for some time that certain bacteria reduce Fe(III) in various geochemical environments. Microbial Fe(III) reduction has been observed primarily in low temperature environments that have been extensively influenced by modern surface biogeochemical processes such as weathering or microbial metabolism. It is also known that certain bacteria such as Desulfovibrio desulfuricans reduce sulfate to sulfide under anaerobic conditions. The formation of some mineral deposits such as magnetite deposits in banded iron formations in both ancient and modern times may be attributed to the action of such bacteria.
Some magnetotactic bacteria have been reported to form magnetic nanocrystals within the cell. However, the reported ratio of product nanocrystals to biomass is relatively low, typically a few nanocrystals per cell.
Several varieties of thermophilic bacteria such as Thermoanerobacter and Thermoanerobium are known to reduce Fe(III) ions as part of their respiration processes. Applicants' co-pending U.S. patent application Ser. No. 10/174,184 discloses the use of these microorganisms to create novel mixed metal oxide compositions in which at least one of the metals is reducible by the bacteria and in which other metal(s) may or may not be reducible by the bacteria.
In U.S. Pat. No. 6,444,453 methods are taught for using bacterial reduction to make doped or mixed metal oxides, particularly ferrites. A suspension of amorphous Fe(III) oxyhydroxide is used as the source of iron and the dopants are added as various soluble species in the culture medium. Specifically, Zn is added as ZnCl2, Co added as CoCl2 or as Co(III)-EDTA, and Cr is added as K2CrO4. Those skilled in the art will appreciate that the method taught in '453 will be limited in some cases by the potential toxicity of these soluble species to the bacteria. Toxicity may preclude the use of some dopants completely and in other cases may limit the overall productivity by limiting the usable concentration of a particular dopant in solution to some maximum that is tolerable by the bacteria.